In recent years, as a new medical treatment for cancers, a charged particle beam treatment which irradiates a charged particle beam on an affected area of a specimen has been expected. Heavy ions represented by, for example, a proton and a carbon ion are included in charged particles to be used for the medical treatment.
Dose of common radiation ray such as X-ray or γ-ray is high in a portion just under a body surface and gradually decreases according to a depth from the body surface. On the contrary, dose of a charged particle beam is low in a portion close to the body surface and rapidly increases to achieve a peak at a given depth (generally, the peak is called “Bragg peak”), and the charged particle beam does not penetrate beyond the peak. By regulating a position and height of the Bragg peak to irradiate a tumor according to a shape thereof, it becomes possible to take a shot at a cancer lesion.
For generating a charged particle beam, particle are ionized by an ion source and accelerated by an accelerator such as synchrotron by giving energy to the particles. For example, in the case of generating a carbon ion beam, carbon ions are accelerated to a speed 84% of the speed of light in the accelerator. Then, the accelerated charged particle beam is transported through a beam transport channel to irradiate a specimen by an irradiation means.
Generally, a size of an affected area of a specimen is larger than a beam diameter of a charged particle beam generated by an accelerator. Therefore, as a method for uniformly irradiating a whole affected area with the charged particle beam, a method (Wobbler method) that irradiates the affected area with the charged particle beam by enlarging a beam diameter and a method (spot scanning method) that dispersedly irradiates the affected area by three-dimensionally scanning an irradiation spot without enlarging the beam diameter have been used.
In the both methods described above, an irradiation depth of the beam is regulated according to a position and shape of the affected area of a specimen. A regulation of the irradiation depth can be achieved by controlling energy of the charged particle beam.
There are two methods for controlling the energy of the charged particle beam.
One method is to control an energy supplied to the charged particles by controlling an output of the accelerator. However, this method is not easy since there are many components to be controlled in the accelerator, and further, an accurate control is required for each of the components.
The other energy control method is to attenuate the energy of the charged particle beam by setting a material, which is called a range shifter, for absorbing the energy of the charged particle beam on a beam axis (for example, see patent literature 1). The method which uses the range shifter can easily control the energy of the charged particle beam, compared with the method which controls the output of the accelerator.    Patent literature 1: Japanese Patent Laid-open Publication No. 2001-562
However, when the energy of the charged particle beam is controlled by using the range shifter, a beam diameter of the charged particle beam is enlarged along a traveling direction as a secondary effect after passing through the range sifter. That is, the beam diameter is varied depending on an irradiation depth in the specimen. As a result, a treatment planning has been difficult. Especially, in the spot scanning method, three-dimensional filling out of the affected area of the specimen with a uniform irradiation spot becomes difficult.
Therefore, to solve the issue described above, there has been a demand for an irradiation filed forming device which can form an irradiation field with a constant beam diameter, while regulating an irradiation depth of the charged particle beam.